1. Field of the Invention
This invention relates generally to the art of organic electronic devices. More specifically, the invention relates to Organic Light Emitting Diode devices.
2. Related Art
Display and lighting systems based on LEDs (Light Emitting Diodes) have a variety of applications. Such display and lighting systems are designed by arranging a plurality of photo-electronic components (“components”) such as arrays of individual LEDs. LEDs that are based upon semiconductor technology have traditionally used inorganic materials, but recently, the organic LED (“OLED”) has come into vogue for certain applications.
An OLED is typically comprised of two or more thin at least partially conducting organic layers (e.g., an electrically conducting hole transporting polymer layer (HTLs) and a light emissive polymer (LEP) layer) which are sandwiched between an anode and a cathode. Under an applied forward potential, the anode injects holes into the conducting polymer layer, while the cathode injects electrons into the emissive polymer layer. The injected holes and electrons each migrate toward the oppositely charged electrode and form an exciton in the emissive polymer layer. The exciton relaxes to a lower energy state by emitting a photon.
The color of light emission from such a device structure is controlled by emission properties of the LEP layer. For example, white emission can be achieved by blending a blue-emitting LEP with polymers (or small molecules) that emit in green and red regions of spectrum (see e.g. J.-I. Lee et al, Optical Materials 21, 205-210 (2002) and Y. Kawamura et al, Journal of Applied Physics 92, 87-93 (2002)). In this case direct carrier trapping and/or energy transfer from the blue host to the red and green dopants will redistribute emission between blue, green and red chromophores thus resulting in white emission. A similar approach is to synthesize a copolymer incorporating all three types of chromophores in one polymer chain thus preventing possible phase separation that may occur in a blend.
However the above approaches have several drawbacks. Doping with emitting chromophores not only changes the emission spectrum but can also result in undesirable changes in charge transport (e.g. due to trapping of charges) properties of the host LEP. In addition to that, as only very small concentrations of emitting dopants are required to change the color of emission, it could be difficult to precisely control relative concentrations of the dopants in order to achieve desirable and reproducible emission color.
Organic LEDs based on small molecule materials (SMOLEDs) offer several advantages over the PLEDs as far as the fabrication of white emitting OLEDs is concerned. Apart from doping one emitting layer with different chromophores, as in the PLED approaches above, white light emission can be achieved by fabricating multilayer structures, the approach that can be easily implemented by sequential vacuum deposition of required organic layers (J. Kido et al, Science 267, 1332 (1995)), emitting in different regions of the visible spectrum.
The possibility to fabricate multi-layer SMOLED structures due to the flexibility of vacuum deposition technique cannot be easily employed in PLEDs, which are solution processed. In order to be able to fabricate a multilayer polymer structure, a subsequent polymer layer must be coated using the solvent that does not dissolve the underneath layer onto which the above layer is coated.
One of the approaches is to prepare the first polymer layer via a precursor route whereby a soluble precursor is first spin-coated and then thermally converted to a luminescent polymer insoluble in any of the commonly used solvents. Afterwards a second electroluminescent polymer layer can be applied by spin-coating on top of the first layer. This way a multilayer polymer structure can be realized and emission from both polymer layers can be achieved (R. H. Friend et al, U.S. Pat. No. 5,807,627 (1998)). And generally, if a solvent used for the subsequent layer does not dissolve the previous layer, then a multilayer structure can be prepared. However, this approach is limited in which materials can be used to create the layers. Emission spectrum of such a device is determined by the bandgaps of the emitting polymers constituting each layer and is not readily tunable, since tuning the emission spectrum would involve syntheses of a new polymer.
It would be advantageous to design and fabricate a device structure that can utilize a wide variety of polymers to produce a white emitting OLED device or display.